1. Field of the Invention
This invention relates to prostheses for replacing intervertebral discs of the human spine and more particularly to endplates for intervertebral disc prostheses having a central domed region and a peripheral rim. The invention also relates to surgical instruments for preparing a vertebra to receive an intervertebral disc prosthesis.
2. Background Art
The most common pathologic condition causing chronic low back pain and neck pain is degenerative disc disease. Disc excision, decompression, and/or spinal fusion have been surgical procedures commonly performed on patients with degenerative disc disease when non-operative treatment fails to relieve chronic disabling pain. More recently, replacement of a degenerated disc with an artificial disc prosthesis has become an available choice for pain relief and restoring function.
For successful results from disc arthroplasty, it is desirable that a prosthesis for implantation in the intervertebral space include structures disposed and configured to promote the proper positioning, alignment and immediate post-operative stability of the prosthesis within the intervertebral space. Post-operative stability is important since a disc prosthesis that is not securely stabilized within the disc space may result in accelerated disc degeneration, the formation of osteophytes, and subsidence into the vertebrae, leading to a loss in disc height. The occurrence of such subsidence is in turn affected by three factors: (i) the applied load upon the device, (ii) the quality, or bone mineral density, of the vertebra and (iii) the contact surface area between the prosthesis and the bone. The vertebral endplates of the normal human lumbar-sacral spine have a wide range of variable contours and curvatures. However, in general, they have a concave curvature, with a rim of dense compact or cortical bone and a central region of cancellous bone.
Accordingly, flat prosthetic endplate designs are prone to have problems of incongruous fit between the prosthetic endplate and the vertebral endplate due to mismatched shape, and may not provide post-operative stability of the prosthesis in the disc space. A number of disc prostheses having flat endplates incorporate additional mechanisms of fixation to bone by screws, spikes, keels, or serrated surfaces. Some of such mechanisms may require excessive distraction of the adjacent vertebrae in order to provide sufficient intervertebral disc space for insertion. Such a procedure may damage adjacent tissues or even result in fracture of the vertebral bone.
Other prosthesis endplate designs having somewhat convex surfaces may provide a better match for the generally concave contours of the vertebral endplates. However, the terminal surfaces of the vertebrae do not have a simple geometrical shape, and vary from patient to patient. Various methods of shaping the vertebral endplates to accommodate the matching endplates of an intervertebral prosthesis have been proposed. However, a need has continued to exist for an intervertebral prosthesis endplate that can be readily fitted to the vertebrae of a patient with simple and accurate adaptation of the prosthesis endplate to the vertebral endplate, thereby providing a relatively large contact area between the prosthesis endplate and the vertebral endplate with concomitant good stability of the prosthesis within the intervertebral space.
Proper preparation of the vertebral endplates during disc replacement surgery is an important element of the procedure that can affect the clinical outcome of the surgery. It is desirable that the instruments provide an accurate means of shaping the endplates to receive the implant without damaging adjacent structures. Various instruments have been disclosed or utilized, but many suffer from the disadvantage that they damage or remove some of the peripheral bony rim in order to reach the endplate. The result is that the implant receives less than optimum support from the remaining compact bone, leading to higher interfacial contact stresses and an increased potential for expulsion. It is also desirable that the instruments for preparing the vertebral endplates be easy to use.
Accordingly, a need has also continued to exist for instruments and procedures for preparing a seat in the vertebral endplate to receive a complementary structure on the prosthesis endplate.